Perspectives for dye-sensitized nanocrystalline so

PerspectivesforDye-sensitizedNanocrystallineSolarCellsMichaelGraÈtzel*LaboratoryforPhotonicsandInterfaces,SwissFederalInstituteofTechnology,CH-1015Lausanne,SwitzerlandThedye-sensitizedsolarcells(DYSC)providesatechnicallyandeconomicallycrediblealternativeconcepttopresentdayp±njunctionphotovoltaicdevices.Incontrasttotheconventionalsystems,wherethesemiconductorassumesboththetaskoflightabsorptionandchargecarriertransport,thetwofunctionsareseparatedhere.Lightisabsorbedbyasensitizerwhichisanchoredtothesurfaceofawidebandgapsemiconductor.Chargeseparationtakesplaceattheinterfaceviaphoto-inducedelectroninjectionfromthedyeintotheconductionbandofthesolid.Carriersaretransportedintheconductionbandofthesemiconductortothechargecollector.Theuseoftransitionmetalcomplexeshavingabroadabsorptionbandinconjunctionwithoxide®lmsofnanocrystallinemorphologypermitstheharvestingofalargefractionofsunlight.Near-quantitativeconversionofincidentphotonsintoelectriccurrentisachievedoveralargespectralrangeextendingoverthewholevisibleregion.Overallsolar(standardAM1.5)toelectricconversionecienciesover10%havebeenreached.Therearegoodprospectsforproducingthesecellsatalowercostthanconventionaldevices.Herewepresentthecurrentstateofthe®eld.Weshalldiscussnewconceptsofthedye-sensitizednanocrystallinesolarcell(DYSC),includingsolidheterojunctionvariants,andanalyzetheperspectivesforfuturedevelopmentofthetechnologyintothenextmillennium.Copyright#2000JohnWiley&Sons,Ltd.INTRODUCTIONPhotovoltaicdevicesarebasedontheconceptofchargeseparationataninterfaceoftwomaterialsofdi€erentconductionmechanism,normallybetweensolid-statematerials,eithern-andp-typeregionswithelectronandholemajoritycarriersinasinglesemiconductormaterial,hetero-junctionsbetweendi€erentsemiconductorsorsemiconductor±metal(Schottky)junctions.Inphoto-electrochemicalcells,thejunctionsaresemiconductor±electrolyteinterfaces.Inrecentyears,despiteprolongede€ort,adisillusionhasgrownabouttheprospectsofelectrochemicalphotoe€ectsattheseinterfacesgivingrisetocompetitivephotovoltaicdevices,sincethosesemiconductorswithbandgapssucientlynarrowforecientopticalabsorptionofvisible-lightphotonsarenecessarilyinsucientlystableagainstphotocorrosion.Thewidthofthebandgapisameasureofthechemicalbondstrength.Thesemiconductorsstableunderillumination,typicallytheceramicoxidesofreactivemetalssuchastitanium,thereforehaveawidebandgap,anabsorptionedgetowardstheultravioletandaconsequentinsensitivitytothevisiblespectrum.Abreakthroughrepresentedbytheseparationoftheopticalabsorptionandthechargeseparationprocessesinphoto-electrochemistry,wasrealizedbytheassociationofaredoxdyeasCopyright#2000JohnWiley&Sons,Ltd.Received10June1999Revised30September1999PROGRESSINPHOTOVOLTAICS:RESEARCHANDAPPLICATIONSProg.Photovolt.Res.Appl.8,171±185(2000)MillenniumSpecialIssue*Correspondenceto:MichaelGraÈtzel,LaboratoryforPhotonicsandInterfaces,SwissFederalInstituteofTechnology,CH-1015Lausanne,Switzerland.light-absorbingmaterialwiththewidebandgapsemiconductor.1Thispatent-protectedconceptisnowwellknowninprincipleandhasbeenlicensedforindustrialdevelopment.2±6Thisattentionisevidenceoftheestablishedcredibilityoftheprinciple,anditsprogresstowardspracticalapplications.7±13Inthefollowing,theoperationalprinciplesoftheDYSC,thehistoricaldevelopmentofdye-sensitizedphoto-voltaicsanditscurrentstatuswillbebrie¯ydiscussed.Themainpartofthepaperwillgiveperspectivesontheissuesandopportunitiesassociatedwiththistechnologyforthenext20±50years.OPERATIONPRINCIPLEOFTHEDYE-SENSITIZEDNANOCRYSTALLINESOLARCELL(DYSC)AschematicpresentationoftheoperatingprinciplesoftheDYSCisgiveninFigure1.Attheheartofthesystemisamesoporousoxidelayercomposedofnanometer-sizedparticleswhichhavebeensinteredtogethertoallowelectronicconductiontotakeplace.ThematerialofchoicehasbeenTiO2(anatase),althoughalternativewidebandgapoxidessuchasZnOandNb2O5havealsogivenpromisingresults.14,15Attachedtothesurfaceofthenanocrystalline®lmisamonolayerofthechargetransferdye.Photo-excitationofthelatterresultsintheinjectionofanelectronintotheconductionbandoftheoxide.Theoriginalstateofthedyeissubsequentlyrestoredbyelectrondonationfromtheelectrolyte,usuallyanorganicsolventcontainingredoxsystem,suchastheiodide/triiodidecouple.Theregenerationofthesensitizerbyiodideinterceptstherecaptureoftheconductionbandelectronbytheoxidizeddye.Theiodideisregeneratedinturnbythereductionoftriiodideatthecounterelectrode,thecircuitbeingcompletedviaelectronmigrationthroughtheexternalload.Thevoltagegeneratedunderilluminationcorrespondstothedi€erencebetweenthequasi-Fermileveloftheelectroninthesolidandtheredox(Nernst)potentialoftheelectrolyte.Overall,thedevicegenerateselectricpowerfromlightwithoutsu€eringanypermanentchemicaltransformation.Acceleratedendurancetestshaveshownthatrutheniumbasedchargetransfersensitizerscansustain100millionturnoverswithoutdecomposition,Figure1.Principleofoperationandenergylevelschemeofthedye-sensitizednanocrystallinesolarcell(DYSC)photoexcitationofthesensitizer(S)isfollowedbyelectroninjectionintotheconductionbandofthemesopor